1. Field of the Invention
The present invention relates to an ink, an ink cartridge and an ink jet recording method.
2. Description of the Related Art
In recent years, an ink used for an ink jet recording method has been required to still more improve the image density of a recorded image. Among recording media on which an image is recorded, various kinds of plain paper, which are different in ink permeability, are available, and the difference in ink permeability affects image characteristics. In particular, a recording medium high in ink permeability tends to lower the image density of an image formed. In recent years, the ink jet recording method has been widely used; accordingly, an image formed thereby has been required to achieve a high image density irrespective of the kind of the recording medium including such a recording medium high in permeability.
In order to meet the above requirement, there are proposals for improving printed letter quality and image density by an ink containing a self-dispersible pigment to the surfaces of particles of which a functional group such as a carboxylic acid group is bonded and a salt (see Japanese Patent Application Laid-Open No. 2000-198955, Japanese Patent Application Laid-Open No. 2008-001891 and Japanese Patent Application Laid-Open No. 2002-080763). Japanese Patent Application Laid-Open No. 2008-001891 and Japanese Patent Application Laid-Open No. 2002-080763 describe that the image density of a recorded image can be improved by more increasing the density of the functional group on the surfaces of the pigment particles. There is also a proposal for improving the image density by an ink containing a self-dispersible pigment to which a functional group high in reactivity to calcium is bonded on the basis of a calcium index that is an index of reactivity to calcium (see Japanese Patent Application Laid-Open No. 2009-515007). Japanese Patent Application Laid-Open No. 2009-515007 discloses a self-dispersible pigment to the surfaces of particles of which a phosphonic acid group is bonded.